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Jump to first page Lecture 4 n Below ground processes n Water reaches the ground beneath a forest as throughfall (direct raindrop passage through the canopy,

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Presentation on theme: "Jump to first page Lecture 4 n Below ground processes n Water reaches the ground beneath a forest as throughfall (direct raindrop passage through the canopy,"— Presentation transcript:

1 Jump to first page Lecture 4 n Below ground processes n Water reaches the ground beneath a forest as throughfall (direct raindrop passage through the canopy, or by dripping of intercepted water from the canopy) and stemflow. n Various amounts of litter interception may follow throughfall or stemflow.

2 Jump to first page Remember the 3 partitioning points! With infiltration we are focussing on this point.

3 Jump to first page Infiltration n Infiltration is the process by which water moves from the surface of the mineral soil into spaces within the soil itself. n The rate at which water enters the soil from the surface is a function of water-input rate and infiltration capacity (i.e. the maximum rate that soil will accept water) n How do forests affect infiltration ?

4 Jump to first page Forest litter reduces the mechanical effect of the water drops, which tends to destroy the surface soil structure. Litter also forms a “sponge- like” humus layer allowing slow movement of water into the soil. Better soil structure maintains large stable pores in the soil through which water can pass. Activities of the different living organisms (microorganisms, animals, roots) increase the porosity and stabilize the structure of the soil. For these reasons, infiltration rates are usually higher underneath forest canopies Higher infiltration rates mean less surface runoff and delayed runoff response.

5 Jump to first page This a a typical soil structure under an a well established forest

6 Jump to first page Litter Fall n Litter fall rates are important when the forest nutrient cycle needs to be understood.

7 Jump to first page Infiltration - Effects of Site Preparation n Site preparation is a very important component of commercial forestry silvicultural practices. n Represents a major disruption of the soil. n Extent to which this affects hydrological processes depends upon the intensity and method used.

8 Jump to first page Infiltration - Effects of Site Preparation n Impacts on various hydrological processes u Increase in infiltrability - higher proportion of rainfall enters soil than under normal conditions. u Disrupted soil structure results in rapid redistribution of water from top- to sub-soil. u Disrupted soil structure results in more rapid and denser root development u Tilling has been shown to increase porosity (i.e. saturation content of soil).

9 Jump to first page n Deep ripping above Himeville, KwaZulu- Natal before establishment of pinus patula.

10 Jump to first page Soil moisture in the root zone is very important.

11 Jump to first page Soil Water n Important terms to note are: n Soil-Water Content u amount of water in the soil (volumetric and gravimetric) - quantative n Soil-Water Potential u the availability of the water to plants (largely qualitative) n Methods of soil water content measurement include u direct measurement by gravimetric methods (oven or microwave drying) u indirect measurements by neutron probes, capacitance probes, time domain reflectometry (TDR), tensiometers, etc.)

12 Jump to first page Soil Moisture and Transpiration n In the soil, the maximum amount of water (mm.m -1 of the soil) that is available for transpiration depends on the texture and depth of the soil n The more the available reserve is exhausted, the more difficult it is for the trees to extract water from the soil. n Different trees reduce their transpiration at different soil moisture contents (the CONST value in ACRU) n The depth up to which water is extracted corresponds to the rooting depth (excepting the capillary activity which is difficult to quantify) n Initially water is almost exclusively extracted from the surface layers; with drying soils, the trees progressively consume water from lower reserves.

13 Jump to first page Soil Water n TDR – DEMO

14 Jump to first page The Neutron Probe is a device used to measure soil moisture content

15 Jump to first page A radioactive source emits fast and slow neutrons The fast neutrons collide with elements and slow down. Hydrogen is most effective in slowing down the neutron. A detector counts the number of slow neutrons returned to the source

16 Jump to first page Soil Water  Advantages and disadvantages  It measures a sphere of about 30 cm in diameter  A calibration curve or equation relates neutron count to water content  Background H, bulk density, and other chemical components may influence the measuring results  Radioactive

17 Jump to first page Soil Moisture Trends Note how observed soil moisture is progressively drier. However, the model has limitations….. Results from soil moisture monitoring beneath eucalyptus at Ntabanhlope. The trees were established in March 1991.

18 Jump to first page Grassland These are the results of a year of soil moisture measurements beneath grassland, Wattle (a. mearnsii), Pine (p. Patula) and Eucalypts (e. grandis) near Greytown. Neutron probe measurements were taken weekly and results are averaged to give results for 300mm, 800mm and 1300mm below the soil surface.

19 Jump to first page Grassland Eucalyptus Wattle Pine At 300mmm, the grassland site has the lowest, and the eucalypt and pine sites have the highest soil moisture. At 1300m, the grassland site has much higher soil moisture than the trees, and the eucalypt site has the lowest soil moisture. Exercise: Provide an explanation for the trends identified in the previous two points.

20 Jump to first page Rooting pattern - Pine

21 Jump to first page Rooting pattern – Pine

22 Jump to first page Rooting pattern - Eucalypt

23 Jump to first page Rooting pattern - Wattle

24 Jump to first page As the trees consume water, less moves downward and downslope to groundwater and rivers and streams. Soil moisture deficits increase. Soil Moisture – influence on water resources

25 Jump to first page Useful Web Sites n


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